How To Fire Child Web Control Event in Parent Web Control in ASP.NET

Consider two web parts "BooKTicket.ascx" (Parent web part) and "Passenger.ascx" (Child web part). Passenger.ascx has a RadioButtonList and on selected index changed event of that control, I need to perform some activity in the parent web part.

<%-- Passenger.ascx --%>
<div class="passenger-number-holder row form-group" style="padding-top:5px">
        <div class="col-md-3">
            <asp:Label ID="lblBoardingPass" runat="server" Text="Boarding Assistance Required" AssociatedControlID="ddlTravelPass" />
        </div>
        <div class="col-md-6">
            <asp:RadioButtonList ID="rdoListBoardingAssistance" runat="server" RepeatDirection="Horizontal" AutoPostBack="true" OnSelectedIndexChanged="rdoListBoardingAssistance_SelectedIndexChanged" OnPreRender="rdoListBoardingAssistance_PreRender">
                <asp:ListItem Value="1">Yes</asp:ListItem>
                <asp:ListItem  Selected="True" Value="0">No</asp:ListItem>

            </asp:RadioButtonList>
        </div>

    </div>

//Passenger.ascx.cs

//Create a Event Handler
public event EventHandler rdoUpdated;

//Call the evnt handler on SelectedIndexChanged event of RadioButtonList
 protected void rdoListBoardingAssistance_SelectedIndexChanged(object sender, EventArgs e)
 {
    rdoUpdated(this, EventArgs.Empty);
 }

<%-- BookTicket.ascx --%>
<%@ Control Language="C#" AutoEventWireup="true" CodeFile="BookTickets.ascx.cs" Inherits="Day3.VLine.Website.WebParts.CMSWebParts_VLine_JourneyPlanner_BookTickets" %>
<%@ Register Src="~/CMSWebParts/VLine/JourneyPlanner/Passengers.ascx" TagPrefix="line" TagName="Passengers" %>

<asp:UpdatePanel ID="udpUpdate" runat="server" ChildrenAsTriggers="false" UpdateMode="Conditional">
  <ContentTemplate>
    <%-- Hook up the event OnrdoUpdated --%>
    <line:Passengers runat="server" ID="Passengers" OnrdoUpdated="Passengers_rdoUpdated"  />
    <asp:Button ID="btnNext" runat="server" Text="Next"/>
  </ContentTemplate>
</asp:UpdatePanel>
//Add this method in BookTicket.ascx.cs
protected void Passengers_rdoUpdated(object sender, EventArgs e)
{
    UpdateButtonTextOnChange("passenger");
}

private void UpdateButtonTextOnChange(string sender)
{
       if(sender == "passenger")
       {
            btnNext.Text = "Next - Updated Text";
            udpUpdate.Update();
       }

 }